This invention relates in general to tensioning of risers used in offshore oil production and exploratory drilling and, more specifically, to an improved tensioning ring and slip assembly for fastening a riser tensioning apparatus to a riser pipe suspended from an offshore drilling vessel or floating platform. Off shore oil drilling and production operations are conducted through a pipe, called a riser, running from the subsea wellhead to the surface platform or floating vessel. In order to support the weight of these risers and to control the stresses induced by ocean currents and vessel motions, the upper end of the riser is connected to a tensioning device. This riser tensioner maintains a predetermined range of tension throughout the range of vertical and lateral motions of the drilling or production rig. The conventional approach to tensioning risers is to use a combination of a hydraulic or pneumatic mechanical cylinder, pressurized using a compressed gas, to apply the tensioning forces to the riser. Each riser tensioner is located on a deck of the floating platform or floating vessel and structurally connected through its cylinders to the riser. The cylinders are typically connected to the risers through cylinder rods. The rods are connected to a tension ring which may be threaded directly onto, or otherwise connected to, a slip assembly surrounding and fastened to the riser pipe. The tension ring and/or slip assembly generally is movable along the riser to permit positioning in an optimum position with the cylinders extended in an optimum manner. Where the tension ring is threaded onto the riser, this adjustment is accomplished by rotating the tension ring. Typical of such systems is that disclosed by Myers et al in U.S. Pat. No. 4,883,387, which uses a conventional thread or spiral groove on a riser top joint engaging a tensioner ring.
Because of the high tension between base and tensioning ring during normal operation, rotating the ring or slip assembly to thread the ring up or down along the riser is often very difficult. The ring or slip assembly and riser are often jammed together with tight threads so that even when the tension is released rotation of the ring is very difficult.
The thread forms used are subject to deformation which increases the difficulty of rotation. Further, tight machining tolerances for the threads are required to prevent further problems in rotating the ring. A great variety of thread forms have been developed for widely varying, specific, purposes. For example, rounded threads such as are described by Johansson et al in U.S. Pat. No. 3,645,570 have been developed for percussion drill rods. Rounded thread bottoms such as are described by Frerejacques in U.S. Pat. No. 4,861,210 have been designed to reduce stress. Non-symmetrical threads are disclosed by Ditson in U.S. Pat. No. 4,063,837 for connecting lengths of drill rod together. The prior thread forms, however, do not address the problems which occur in tension ring assemblies.
Therefore, there is a continuing need for tension ring and/or slip assemblies for riser tensioning systems that operate well in the difficult environment, permit ease of rotation and ease of positioning when tension is released, resist deformation under tension and do not require tight manufacturing tolerances.